Wandering Dread, The Headless Horseman

Death's Might. All spells deal 40 (4d12) necrotic damage on hit. Spells can be cast as a bonus action.

Spellcasting. The Headless Horseman is a 9th-level spellcaster. Its spellcasting ability is [Intelligence] (spell save DC 18, +10 to hit with spell attacks). The Headless Horseman has following Wizard spells prepared:
Cantrips (at will): Eldritch Blast, Burning Hands
Spells (30 slots): Fireball, Thunderwave

Death's Edge. Any character that falls unconscious while in combat with this monster instantly dies, making no death saving throws.

Malevolence. Enemies within a 60 ft. radius have a -5 modifier to all saving throws.

Inevitable. This monster has three actions per turn, and can use a bonus action with each action.

Unslayable. Immune to the spell Wish and all other instant kill effects. Cannot be removed from its current plane of exhistance.

Harvest. Melee Weapon Attack: +15 to hit, reach 20 ft., one to three targets. Hit:  60 (5d12 + 12) [Necrotic] damage. Can be used as a bonus action.

Reaper's Spiral. Melee Weapon Attack: +12 to hit, 20 ft radius AoE centered on monster., Hit:  45 (3d12 + 15) [Necrotic] damage. Can be used as a bonus action.

Shackled Grave. Class Feature: Selects up to five living enemy characters within 60 ft. The targets must pass a DC 20 constitution save or be silenced and immobilized for one turn.

Oblivion. Class feature:  Selects one character within 60 ft. The target must pass a DC 25 constitution save, or die instantly. This ability can be used once every two turns.

Death Row. Melee charge attack:  Picks a direction, charges in a straight line, up to 60 ft. Targets in the path of the charge take 40 (3d12 + 10) [Necrotic] damage, and are pulled to the end of the charge. Can be used as a bonus action.

Harvest. Melee Weapon Attack: +15 to hit, reach 20 ft., one to three targets. Hit:  60 (5d12 + 12) [Necrotic] damage. Activates whenever a living character moves within range. 

Vengeance. Ranged Attack: +20 to hit, range 60 ft., one target. Hit: 30 (3d12+ 5) [Necrotic] damage. Activates the first time this monster is struck by a ranged attack each turn.

Upon turn beginning restores one legendary action use, legendary actions can be used twice per turn. Has a maximum of 3 stored legendary actions.

Death's Efficacy (Costs 1 Action). Instantly removes periodic damage effects, and heals The Headless Horseman for half of its missing health.

Wailing Hosts (Costs 1 Action). Raises 3+1 per living entity within 90ft. Zombies with 150 additional hit points and  doubled movement speed. Can only be used while no Zombies are alive.

Life-ender (Costs 2 Actions). The next spell or attack used by The Headless Horseman deals 50 (3d12 + 15) [Necrotic] damage. 

Hades' Gates (Costs 2 Actions). The Headless Horseman can instantly teleport up to 60 ft. dealing 20 (2d12 + 9) [Necrotic] damage and 20 (2d12 + 3) [Fire] damage in a 10 ft. AoE around its destination.